Re: [PATCH 1/6] posix-cpu-timers: Fix rearm racing against process tick

[Frederic Weisbecker <frederic@kernel.org>]

On Fri, Jun 04, 2021 at 01:31:54PM +0200, Frederic Weisbecker wrote:
> Since the process wide cputime counter is started locklessly from
> posix_cpu_timer_rearm(), it can be concurrently stopped by operations
> on other timers from the same thread group, such as in the following
> unlucky scenario:
> 
>          CPU 0                                CPU 1
>          -----                                -----
>                                            timer_settime(TIMER B)
>    posix_cpu_timer_rearm(TIMER A)
>        cpu_clock_sample_group()
>            (pct->timers_active already true)
> 
>                                            handle_posix_cpu_timers()
>                                                check_process_timers()
>                                                    stop_process_timers()
>                                                        pct->timers_active = false
>        arm_timer(TIMER A)
> 
>    tick -> run_posix_cpu_timers()
>        // sees !pct->timers_active, ignore
>        // our TIMER A
> 
> Fix this with simply locking process wide cputime counting start and
> timer arm in the same block.
> 
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> Cc: Oleg Nesterov <oleg@redhat.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Eric W. Biederman <ebiederm@xmission.com>

Fixes: 60f2ceaa8111 ("posix-cpu-timers: Remove unnecessary locking around cpu_clock_sample_group")
Cc: stable@vger.kernel.org